Conventional methods of rock breakage, including drilling-and-blasting, ripping and crushing have several disadvantages.
The conventional drill-and-blast technique has the disadvantage of noise, gases, dust and flying debris, which means that both men and machines must be evacuated from the working face. Further disadvantages of the drill-and-blast technique are overbreak, which entails costly reinforcement of the tunnel wall in certain cases, and the obvious danger of storing and handling explosives in a confined working space.
Conventional crushing techniques are also inefficient in that the the rock is made to fail in compression whereas it is weaker and would fail more easily in tension. Consequently, as a result of the large forces required to crush the rock, tool wear is significant, particularly in hard or abrasive rocks.
During the last decade serious attention has been given to replacing the drill and blast techniques for tunnelling, mining and similar operations. One alternative technique involves the use of high velocity jets of water or other liquid to fracture the rock or ore body and numerous devices intended to produce pulsed or intermittent liquid jets of sufficiently high velocity to fracture even the hardest rock have been suggested. Devices of that type are disclosed in for example U.S. Pat. Nos. 3,784,103 and 3,796,371. As yet, however, jet cutting techniques are still unable to compete with the traditional methods of rock breakage such as drill and blast in terms of advance rate, energy consumption or overall cost. Moreover serious technical problems such as the fatigue of parts subjected to pressures as high as 10 or 20 kbar and excessive operational noise remain.
A second, and even older technique for fracturing the rock and for saturating soft rock formations such as coal with water for dust suppression involves drilling a hole in the rock and thereafter pressurizing the hole with water. This technique is disclosed in for example German Pat. No. 230,082. Low pressure water is continuously delivered into the hole for filling the pores adjacent to the hole, thereby suppressing dust and improving the function of the hole as a pressure water cylinder. When a desired degree of massiveness is obtained the water delivery, i.e. the mass transport, into the hole is increased stepwise. The coal stope cannot absorb this suddenly supplied large amount of water which means that a breaking force arises. The method is inapplicable to hard rock formations because of the restriction in working pressure which can be realized or usefully utilized with conventional hydraulic pumps. It is also difficult to apply in practice in soft crumbling rock or badly fissured rock.